Encephalization is defined as the amount of brain mass related to an animal's total body mass. Quantifying an animal's encephalization has been argued to be directly related to that animal's level of intelligence. Aristotle wrote in 335 B.C. "Of all the animals, man has the brain largest in proportion to his size." Also, in 1871, Charles Darwin wrote in his book The Descent of Man: "No one, I presume, doubts that the large proportion which the size of man's brain bears to his body, compared to the same proportion in the gorilla or orang, is closely connected with his mental powers."